1. Field of the Invention
This invention relates to a process for making a zeolite, preferably a MFI-type structure, most preferably a ZSM-5 MFI zeolite, having germanium in the framework (hereinafter referred to as “Ge-zeolite” or, if the zeolite is ZSM-5 MFI, Ge-ZSM-5). This Ge-zeolite is synthesized without using fluoride compounds.
2. Description of the Prior Art
Zeolite is a crystalline hydrated aluminosilicate that may also contain other metals, such as sodium, calcium, barium, and potassium, and that has ion exchange properties (Encarta® World English Dictionary [North American Edition] © & (P) 2001 Microsoft Corporation). A method for preparing a zeolite comprises (a) preparing an aqueous mixture of silicon oxide and sources of oxides of aluminum; and (b) maintaining said aqueous mixture under crystallization conditions until crystals of said zeolite form.
Synthetic zeolites are normally prepared by crystallization of zeolites from a supersaturated synthesis mixture. The resulting crystalline product is then dried and calcined to produce a zeolite powder. The zeolite powder may be bound for use in certain equipment and process, e.g. fluidized bed reactor.
U.S. Pat. No. 3,702,886 discloses preparation of ZSM-5 zeolite by forming crystals from a solution containing tetrapropyl ammonium hydroxide, sodium oxide, an oxide of aluminum or gallium, an oxide of silica or germanium and water by heating the reaction mixture to a temperature of 100° C. to 175° C. for six hours to sixty days, then cooling to room temperature, separating the solid product with filtering, water washing and drying. The examples and the claims were limited to aluminosilicates.
EP0951444 discloses a synthesis of large crystal zeolites by dissolving an aluminum source in an aqueous caustic solution, such as sodium hydroxide, and then adding this to a mixture of a silica source in water, mixing to homogenize the reaction mixture, heating to the nucleation temperature for crystallization, separation of the product crystals by cooling, filtering, water washing and drying.
U.S. Pat. No. 6,723,300 discloses synthesis of porous crystalline material of trivalent elements such as aluminum, boron, iron, indium and/or gallium and tetravalent elements such as silicon, tin, titanium and/or germanium with hydrofluoric acid as mineralizer instead of alkali hydroxides and with an organic directing agent.
U.S. Pat. No. 5,246,688 discloses production of silica-based MFI zeolites dissolving oxides or hydroxides of the tetravalent and trivalent elements, such as silicon (optionally, with germanium) and aluminum, in hydrofluoric acid, forming a precipitate, separating the precipitate and calcining the precipitate.
U.S. Pat. No. 5,371,307 discloses silica/germanium oxide zeolites prepared substantially in the absence of trivalent elements and in the presence of a structuring agent to promote formation of micropores and to stabilize the zeolite. Mobilizers, such as hydrofluoric acid, may be used to promote dissolution of the reagents which supply the silicon and germanium to be incorporated into the zeolite framework.
U.S. Pat. No. 6,471,941 discloses a crystalline material of a trivalent element such as boron, iron, indium, gallium, aluminum, or a combination thereof and a tetravalent element such as silicon, tin, titanium, germanium or a combination thereof which employs fluorides, in particular HF, as a mineralizing agent in the synthesis.
Synthesis of a MFI-type zeolite with germanium in the framework is disclosed in “Synthesis and Characterization of Ge-ZSM-5 Zeolites, H. Kosslick et al., J. Phys. Chem., vol. 97, p. 5678-5684 (1993). Hydrofluoric acid (HF) is used in the synthesis.
Use of fluorine compounds in the synthesis of the germanium zeolites results in halided catalysts which evolve small amounts of HCl and/or HF when these catalysts are heated at elevated temperatures (e.g., catalyst heat treatment, calcination or process conditions). This produces acid halide gas which reacts with metal present in the process equipment.
It would be advantageous to have a process for making a germanium zeolite-type catalyst which did not use fluorine compounds in the synthesis gel.